Most image sensors include an array of pixels arranged into rows and columns. In most color image sensors, each pixel in the image sensor array is associated with a color filter that passes light with certain wavelength properties to thereby capture a color image. Most color filter arrays include a combination of red, blue and green filters.
Typical color filters have a period of two columns in the horizontal dimension and two rows in the vertical dimension. Hence, the array includes identical units of 2×2 pixels. Typical units include a red filter, a blue filter, and two green filters. The associated pixels may be referred to by the color of the corresponding filter.
A Bayer filter array is one example of a filter array having identical 2×2 pixel units. In a Bayer filter array, the two green pixels are diagonal to one another. When placed in an array having many units, some green pixels are located in otherwise red pixel rows, while the remaining green pixels are in otherwise blue pixel rows. The green pixels located in a blue pixel row may be noted as “Gb,” or “green-B.” Green pixels located in a red pixel row may be noted as “Gr,” or “green-R.”
From the filter's perspective, the two types of green pixels are the same. From the pixel's perspective, however, the two green pixels are distinctly different. Because some of the light diagonally incident on a pixel may have passed through neighboring filters of a different color (a phenomena known as “color cross talk”, explained more fully in previously-incorporated U.S. application Ser. No. 11/107,387), the resulting distortion may be different for the two types of green pixels, which have different neighboring pixels. Moreover, in shared-transistor pixel techniques, the layout of a group of pixels sharing certain common transistors may not be identical. This is yet another source of disparity between the two types of green pixels. Electrical cross-talk between the circuitry of the two types of green pixels and their neighbors may create additional unsymmetrical distortion.
Efforts to normalize distortion levels of the two types of green pixels include:                Process engineering efforts to minimize the height of the color-filter-array and its vertical distance from the active area of the pixels. This serves to decrease the amount of color cross talk described above;        Layout techniques to assure that the geometrical structure, including the environment of the two green pixel types is as identical as possible; and        Calculation of the green disparity as a function of the x and y location of each pixel, and on line application of a correction factor—such as described in previously-incorporated U.S. application Ser. No. 11/107,387.The results of these efforts, however, are less than perfect.        